The purpose of the proposed study is to investigate mortality in Region 11 of Chile from 1950 to 2000. This' region, currently with a population of close to a half million people, experienced a peak exposure to arsenic with a population-weighted average drinking water concentration of 580 micrograms/L, from 1955 to 1970. In contrast, water supplies for the rest of the country mostly contained less than 10 4g/L. Following the installation of arsenic treatment plants, concentrations have gradually been reduced, so that by 1990 the average was less than 50 @ig/l-, the current drinking water standard in much of the world. An investigation of mortality in Region 11 during 1989-1993 indicated that rates for bladder, skin, lung, and kidney cancer were increased compared to the rest of Chile. Bladder cancer mortality was markedly elevated [men, SMR = 6.0, 95 percent confidence interval (CI), 4.8-7.4; women, SMR = 8.2, 95 percent CI, 6.3-10.5] as was lung cancer mortality [men, SMR = 3.8, 95 percent CI, 3.5-4.1; women, SMR = 3.1, 95 percent CI, 2.7-3.7]. It was estimated that arsenic might account for 7 percent of all deaths among those aged 30 years and over. If so, the impact of arsenic on the population mortality in Region 11 of Chile would be greater than that reported anywhere to date from environmental exposure to a carcinogen in a major population. Moreover, the impact may have been even greater in previous years, particularly from arsenic-caused diseases with shorter latencies than cancer. The proposed study will therefore collect and analyze 0 available measurements of arsenic in water, and investigate all causes of mortality from 1950 to 2000, in Region 11. As well as cancer, increased mortality might be expected from non-cancer outcomes including cardiovascular, peripheral vascular and cerebrovascular diseases. The impact of arsenic exposure during childhood on pulmonary disease mortality in young adults, and on childhood cancer mortality, will also be assessed. This study will provide a unique opportunity to investigate arsenic-caused mortality, including latency patterns, in one of the world's most significant environmental toxic exposures.